As disclosed, for example, in Patent Literature 1, a mobile communication system having a vehicle onboard communication terminal has been proposed recently. The vehicle onboard communication terminal is used in a vehicle to wirelessly communicate with another vehicle onboard communication terminal used in another vehicle and a roadside device that is installed on a roadside. Communication between the vehicle onboard communication terminal included in the mobile communication system and the roadside device is referred to as road-to-vehicle communication. Communication between the vehicle onboard communication terminal and another vehicle onboard communication terminal is referred to as vehicle-to-vehicle communication.
Road-to-vehicle communication is used to distribute traffic information generated, for example, by a traffic information center or used to provide billing services for the usage of a toll road or a pay parking lot. Therefore, it is generally necessary to prevent road-to-vehicle communication from being obstructed by vehicle-to-vehicle communication.
Schemes for preventing road-to-vehicle communication from being obstructed by vehicle-to-vehicle communication have been studied. The studied schemes include a configuration for using one carrier frequency for road-to-vehicle communication and using another carrier frequency for vehicle-to-vehicle communication and a configuration for providing a slot for road-to-vehicle communication by using a time-division multiplexing method even when the same carrier frequency is used for road-to-vehicle communication and vehicle-to-vehicle communication.
The inventors of the present application have found the following. According to one of the above-mentioned configurations, the road-to-vehicle communication and the vehicle-to-vehicle communication use different frequencies or slots. Therefore, the road-to-vehicle communication can be established without being affected by the amount of data transmitted by the vehicle-to-vehicle communication.
As the other configuration, the road-to-vehicle communication and the vehicle-to-vehicle communication may use the same carrier frequency, and the roadside device and the vehicle onboard communication terminal may transmit data by the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method. In this instance, too, the roadside device and the vehicle onboard communication terminal both transmit data while observing the use of the carrier frequency (also referred to as a carrier). However, when the number of vehicle onboard communication terminals around the roadside device increases, the time during which the carrier is available decreases. As a result, the roadside device may fail to acquire a data transmission right.